The foaming of plastic parts by means of physical or chemical means is known. Blowing agents are mixed with or dissolved in the plastic, ensuring the production or release of gas in the plastic. However, such methods are limited as regards possibilities of triggering the foaming in a targeted, controlled and also locally limited manner. Until now it has also not been possible to produce discrete, patterned surface structures by foaming.
The foaming of plastic parts can serve various purposes, for example reducing the weight of the part, production of a thermally insulating part, production of sponges and other absorbent foams, production of floats or production of creative elements, ornaments or patterns.
A distinction is currently made between various foaming methods:    1. Physical methods, in which a gas is physically introduced into the molten plastic mass and expanded. The gas bubbles that form lead to foaming of the plastic. A disadvantage of this is the high outlay for apparatus and control engineering. The plastics processing machines, e.g. extruders, have to be converted for the foaming by means of a gas supply at great expense. The adjustment of the gas supply and control in connection with the melting behaviour of the plastic represents a further problem.    2. In the so-called dissolution method, plastic components are released from a solid plastic material by means of suitable solvents. This results in chambers and cavities which lead to a desired weight reduction. However, the method is questionable and problematic if for no other reason than that of environmental protection, as the solvents used, with the plastic components contained therein, pose significant problems of disposal or reprocessing.    3. In the past, CFC-containing products were chiefly used as blowing agents in chemical foaming processes. For reasons of environmental compatibility, these blowing agents are however to be avoided and replaced by other blowing agents. Diazo compounds, N-nitroso compounds, sulphohydrazides, urea derivatives, guanidine derivatives, boron hydride/water systems, carbonates and hydrogen carbonates are increasingly used. A disadvantage of the azo compounds is the production of large quantities of ammonia during degradation and during foam formation, which gives rise to concerns as regards a possible health risk. Many carbonates and hydrogen carbonates decompose without further additives in an uncontrolled manner when the decomposition temperature is reached. This results in uncontrolled foaming, and possibly unwanted discoloration and/or unwanted odour.    4. Foams can be produced very easily from polyurethane (PUR), which are known, amongst other things, as foam rubber, and are used as cleaning sponges, mattress materials or cushions, but also for thermal insulation in buildings, refrigerators, heat and cold storage units as well as for insulating pipe systems. For some time, further areas of application for polyurethane foams have been developed, for example in vehicle construction. Polyurethane foams which are provided for thermal insulation have a closed-pore construction so that the cell gases with their low thermal conductivities remain in the foam cells. In the past, trichlorofluoromethane was frequently used as the cell gas. Because of the ozone-depleting property of this halogenated hydrocarbon, it has however largely been replaced, first by carbon dioxide and then by cyclopentane, with the result that today the foam cells as a rule contain a mixture of approximately 10 to 30% cyclopentane and the remainder carbon dioxide.
Most blowing agents and foaming systems themselves or their reaction products are frequently harmful to the environment or to health and/or pose problems during processing or handling. Such a handling problem can for example be an uncontrollably rapid, exothermic or much too slow gas formation, which can result in either no correct foam formation at all taking place in the plastic parts or the foam structures not meeting the desired requirements, for example due to uneven pore formation, undesirable pore sizes (too large or too small), etc.
In the known systems the foaming takes place throughout the plastic material as a rule. For the production of patterns, writing or other design elements it would be desirable to be able to trigger the foaming in targeted, controlled and also locally limited manner.
In the field of plastic processing or working, the use of NIR/IR radiation (heat radiation) by means of lasers or other radiation sources is known. This radiation is used both for plastic welding and for marking and labelling plastics. NIR/IR radiation sources are also used for the targeted heating of plastic materials, e.g. in the PET bottle production process, so-called PET bottling, or in the drawing of films or in the so-called deep drawing process for the production of cups. The use of NIR/IR radiation to produce heat is very efficient, as the heat energy can be applied in a very targeted manner and with lower losses than for example in conventional convection ovens. It is thus possible to achieve shorter process times and more targeted energy use. In the case of varying material thicknesses of the workpieces to be heated, NIR/IR radiation sources offer significantly better heating possibilities as the output can be regulated better and faster.